Hydroprocessing reactions involve the application of hydrogen to a substrate, usually under elevated temperature and pressure, in the presence of a catalyst with the aim of causing a physical or chemical change in the substrate. Most such hydroprocessing reactions occur in refinery operations where the substrate is a hydrocarbon feedstock.
Conventional hydroprocessing catalysts are generally in the form of a carrier of a refractory oxide material on which hydrogenation metals are deposited, the choice and amount of each component being determined by the end use. Refractory oxide materials usual in the art are amorphous or crystalline forms of alumina, silica and combinations thereof (though for some applications, materials such as titania may be used). These oxide materials can have some intrinsic catalytic activity but often only provide the support on which active metals compounds are held. The metals are generally base or noble metals from Group VIII and Group VIB of the Periodic Table which are deposited in oxidic form during manufacture; in the case of base metals, the oxides are then sulphided prior to use to enhance their activity.
Also proposed in the art for hydrotreating and particularly for use in hydrodesulfurization (HDS), especially deep desulfurisation of diesel fractions, are catalyst compositions which contain refractory oxide material but which are made via co-precipitation. European Patent specification EP-A-1090682 describes one such co-precipitation proposal to prepare a hydrotreating catalyst, which catalyst has various properties including a crystalline phase, such as alpha-alumina, viewed as necessary for high activity and to impart mechanical strength and therefore a longer service life in commercial use.
By co-precipitation, the incorporation of a dispersed metals content into a conventional carrier material is attempted by enabling intimate contact between metals compounds and carrier material and thus enabling the metals to become dispersed through the carrier material before shaping. This contrasts with conventional impregnation techniques where only a small amount of metals deposition is possible since the shaped carrier is already formed and there are diffusional and space limitations for the metal ions or compounds to become dispersed through the catalyst support.
Alternative catalyst forms have been proposed for use in the hydroprocessing of, for example, refinery streams. One such group of catalysts are termed ‘bulk catalysts’. Such catalysts are formed from metal compounds only, usually by co-precipitation techniques, and have no need for a catalyst carrier or support; see for example WO 00/42119 and U.S. Pat. No. 6,162,350. Both publications disclose bulk Group VIII and Group VIb metal catalysts. U.S. Pat. No. 6,162,350 discloses that such catalysts may contain one or more of each metal type, and examples show NiMo, NiW and the most preferred NiMoW bulk catalysts. Two preparation routes are disclosed: one utilising fully dissolved metals and the other utilising contacting and reacting the metals in the presence of a protic liquid, such as water, with the requirement that at least one metal is at least partly in the solid state during the addition, mixing and reaction steps. The materials are said to be essentially amorphous with a unique X-ray diffraction pattern showing crystalline peaks at d=2.53 Å and d=1.70 Å.
The preference in U.S. Pat. No. 6,162,350 (and WO 00/42119) is that no binder is incorporated into the final catalyst composition since the activity of the bulk catalyst composition may be reduced (Column 14, lines 10 to 114). If, however, a binder is to be used the resulting catalyst composition comprises the bulk catalyst particles embedded in the binder with the morphology of the bulk catalyst particles essentially maintained in the resulting catalyst composition (Col. 14, lines 24 to 30). The binder when present is preferably added prior to shaping but can be added at any stage in the catalyst preparation. Examples 12 to 14 show addition of binder prior to shaping; Example 15 shows addition of alumina to form a NiMoW-alumina composition having approximately 8 wt % alumina, the alumina being ‘present during the preparation of the bulk catalyst composition’ but without any detail on how this is done whilst preserving the independent morphology of the bulk catalyst particles. No activity data is provided for the binder-containing catalysts of any of these examples.
The preparation of the trimetallic bulk catalyst particles via the route utilising protic liquid and at least one metal at least partly in solid state form, is also described in WO 00/41810. Here the prepared materials are described as having an XRD diffraction pattern in which the characteristic full width at half maximum value of the peak at 2θ=53.6° (±0.7°) does not exceed 2.5° when the Group VIb metals are molybdenum, tungsten, and, optionally, chromium, or does not exceed 4° when the Group VIb metals are tungsten and chromium, or that of the peak at 2θ=63.5° (±0.6°) does not exceed 4° when the Group VIb metals are molybdenum and chromium, all metals being in their oxidic state. Again binder is not preferred but may be present to provide mechanical strength in the final catalyst composition wherein the bulk catalyst particles essentially still maintain an independent morphology.
In Journal of Catalysis 159, 236-245 (1996), Landau et al compare the HDS activity of conventional NiMo and CoMo catalysts against a precipitated bulk NiMo catalyst and a co-precipitated NiMo-silica catalyst (containing 10.1 wt % SiO2 on an oxide basis) on the sulfur-containing substrates of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT). The co-precipitated catalyst here is made by adding silica powder to a water solution of (fully dissolved) nickel nitrate and ammonium paramolybdate and then introducing ammonium hydroxide as a precipitation agent; the product catalysts were recovered by drying, ie evaporation. The results are variable; for DBT neither the bulk catalyst nor the silica-containing variant demonstrates HDS activity to rival the conventional CoMo catalyst; for DMDBT their HDS activity is better. The article points to optimisation of catalytic sites/structures such as Mo(W)S2 crystals, bulk NiMo particles and the Ni—Mo—Al monolayer, and MoS2 crystal edge planes, in order to obtain effective deep desulfurisation catalysts for petroleum gas oils.
In refinery processes, feedstocks contain a variety of contaminants, the main ones being sulfur and nitrogen. While sulfur reduction has always been desirable, increasingly strict regulations on gas emissions eg from motor vehicles, is driving the need for catalysts which can provide ultra low sulfur fuels. For effective HDS activity, and especially for deep desulfurisation required for environmental reasons, a catalyst must be effective to remove all sulfur compounds, whether simple or complex. Nitrogen contaminants, while often low in amount, can have a severe poisoning effect on catalysts and also adversely affect end product storage stability and quality. The poisoning effect on catalysts is such that a catalyst effective for, eg HDS, of a pure chemical feedstock may be ineffective or short-lived when exposed to an impure refinery feedstock.
Thus, there is particularly a demand for hydroprocessing catalysts for feedstocks having both sulfur and nitrogen contaminants, which catalysts have a significant hydrodesulphurisation activity for both simple and complex sulfur-containing compounds in the presence of nitrogen contaminants but even more desirably also have a high or improved hydrodenitrogenation (HDN) activity.